Mechanisms underlying cell differentiation are important for the normal life of organisms and of great interest to biologists. How two adjacent cells come to differ strikingly even though they are closely related ontogenetically is particularly intriguing. The adjacent mesophyll and bundle sheath cells of maize leaves and the leaves of other C4 plants are models for studying mechanisms that have evolved to make the adjacent cells so different. Among others things, they have different complements of enzyme for photosynthetic carbon fixation. One difference between the two types of cells is that the enzyme ribulose bisphosphate carboxylase/oxygenase, which handles every molecule of carbon dioxide fixed photosynthetically, is present in bundle sheath, but not mesophyll cell of maize leaves. This enzyme is the product of a nuclear gene, rbcS, that encodes the smaller of the two protein subunits of the enzyme and of a chloroplast gene, rbcL, that encodes the larger of the two subunits. This project is directed principally at understanding the molecular basis for the differences in the control of expression of the rbcS gene -m3 in the two adjacent cell types. Expression of rbcS-m3 is stimulated in bundle sheath cells via the red light-driven phytochrome signal transduction system. Expression of rbcS-m3 is suppressed in mesophyll cells principally or entirely through a blue light signal transduction system. Some attention is to be paid to mechanisms of control of expression of a gene for the light harvesting chlorophyll a/b protein of the photosynthetic apparatus which is expressed principally in mesophyll cells -- i.e. in a fashion reciprocal to that of the rbcS gene. The in situ transient expression assay that has been developed for these experiments makes it possible to learn whether a gene is expressed preferentially in one of the other of the two cell types. DNA of modified cloned genes attached to reporter sequences are introduced into cells in situ and the effects of the modifications in transient expression in mesophyll versus bundle sheath cells is determined.